The phosphorescent state of dibenzothiophene

Strucfure, 19 (1973) 419-430 Scientific Publishing Company, Amsterdam -

Journal o~~o~e~alar 0 E&e&r

THE P~OSPH~R~SCE~

MARCEL

Institut

Primed in The Netherlands

STATE OF D~ENZOT~OPHENE

BAIWIR

de Physique,

UrGersite’ de Li&ge uu Sort Tilman, B-4000 Li.Gge (Belgium)

ABSTRACT

The ESR study of phosphorescent o,o’-bridged b~ph~~yls has shown that the nature of the heteroatom has little effect on the molecular properties of the lowest triplet state of these molecules. This observation allows us to assign the symmetry of this state and to propose an electronic scheme for the phosphorescence in this family of aromatic compounds.

INTRODUCTION

We present here a summary of the recent work that we have achieved on the electron spin resonance (ESR) of dibenzothiophene and some of its isologues in their phosphorescent state. Most of this work has been done in collaboration with Drs. J-M, Lhoste, J, Mispelter and J.Ph. Grivet, from the Laboratoire de Biophysique of the Museum National d’Histoire Naturelle in Paris. Mispelter et al. ’ first determped the principal values of the hyperfine coupling tensor of an aromatic #C-3’ fragment by studying the triplet state of fy-and ~-~uoronaphth~ene in a single crystal of durene. They showed that fluorine substitution in aromatic compounds can be described by a “local perturbation model” in which part (about 10%) of the spin density on the adjacent carbon is shifted onto the Zp, fluorine atomic orbital, the other ?I spin densities being unchanged. The same authors proposed a “fluorine labelling method” to map the spin distribution of an aromatic molecule in its lowest triplet state and they applied it to biphenyl 2_ The validity of the “local perturbation model” and of the “fluorine labelling method” was confirmed by the study of phosphorescent biphenyl and Z-~uorobipheny~ in a dibenzofuran single crysM3.4

.

We studied the phosphorescent state of some o,o’-bridged biphenyls, namely carbazole (DBNH), fluorene (DBCH,), dibenzofuran (DBO) and dibenzothiophene -(DBS). We first studied the ESR of the phosphorescent

420

DBS in a DBO single crysta15, and then the bridged biphenyls in glassy solutions of ethanol, using the “fluorine labelling method”4 *6_ The results of all these investigations, together with some spectroscopic arguments, allowed us to propose an assignment of the symmetry of biphenyl and of its bridged derivatives in their lowest triplet state and, then, an electronic scheme of the phosphorescence of these compounds7_ We wish to report here the results concerning DBS, which we have studied more extensively than the other bridged biphenyls for experimental reasons*. We shall discuss first the molecular properties of phosphorescent DBS and then we shall consider it in the scope of the family of bridged biphenyls. Finally, we shall discuss the possible ways of populating and depopulating the lowest triplet state of DBS. As far as experimental procedures are concerned, we shall refer to the papers mentioned above. In an appendix, we shall describe the triplet state of an unknown impurity in DBO and which also seems to be present in other bridged biphenyls. ESR OF PHOSPHORESCENT

DIBENZOTHIOPHENE

Single crystals of DBO doped with DBS (10m2 M initially) were grown from the melt in a nitrogen atmosphere in a Bridgeman furnace. The orientation of the samples relative to the static magnetic field was obtained from the crystallography of the DBO, as obtained from X-ray diffraction9 and polarized microscopy. The molecular axes of the DBS are given in Fig. 1.

=/

X-O,S,NH,CHz

Al

1

1

1

1

~~

i

1

i

i

B1

1

i

i

i

x

82

i

i

i

i

L 4

Fig. 1. Molecular poiint group.

Y RY RZ

axes of the o, o’-bridged

biphenyls

and the character

table of the Czv

421

The description of a triplet molecule in a static magnetic field is usually made using a phenomenological spin-Hamiltonian’ ’ BL=PS-&H+S-&SS

g

S.&I,

c

(S = 1)

which is the sum of anisotropic Zeeman electronic, fine structure and hyperfine structure terms. The & and &-tensors are diagonal in the molecular axes of Fig. 1 and the first two terms of % may be rewritten’ ’ + gNSyHy f- gzSz Hz) -

Bl’ = P&,&H, withX+

Y+Z=O

(XS,2 + ysy” + 23;)

.

From the angular dependence of the resonance fields, we may conclude that the DBS enters the DBO lattice as a true substitutional impurity, the fine structure axes of the guest coinciding with the molecular axes of the host. When the static magnetic field is along the y molecular axis, a splitting of the resonance lines into 13: 1 triplets is observed (Fig. Z), the experimental coupling constant being 9 gauss. In the other two canonical directions, no hyperfine splitting can be resolved. This means that the observed splitting is due to the interaction of the unpaired electron spin with the nuclear spins of the two magnetically equivalent protons 2 and 7.

I

10 gauss

I

Fig. 2. Hyperfine structure of the y-line (low field) phorescent DBS in a DBO single crystal (from ref. 5).

of

the ESR

spectrum

of

phos-

422

The proton hyperfine tensor was derived from the work of McConnell et al.’ 2 on the malonic acid radical. It has been confirmed by the treatment of a conjugated GC-H fragment in the triplet naphthalene by Hirota et al. ’ 3. Its principal values are, in gauss, for a unitary spin density, - 12.0 (along the C-H bond); - 24.6 (normal to the molecular plane), and - 36.0 (normal to the other two directions). In such a description, the spin density in the 7rorbitals of carbon atoms 2 and 7 is equal to 0.255. It must also be noted that the best line shape fit is obtained by assuming that a spin density of 0.10-0.15 is present on another pair of equivalent carbon atoms. Table 1 shows the main parameters of the phosphorescent DBS compared with those of the biphenyl. The following conclusions can be drawn: (i) The numerical values of the zero-field splittings and of the lifetime of the DBS phosphorescent state clearly show that it is a (~,7f*) state. (ii) The similar results for biphenyl and DBS in their lowest triplet state indicates that there must be no conjugation through the heteroatom in the latter molecule. This statement is emphasized by considering the very low anisotropy of the s-tensor and the value of rP , two pieces of experimental evidence which show the very weak influence of the sulphur atom on the internal spin-orbit coupling. (iii) This similarity is a further argument in favour of the planarity of phosphorescent biphenyl, suggested by Wagner from spectroscopic measurementsl 4 and by Mispelter et al. 2 V3from ESR experiments. TABLE

1

&tensorprincipalvalues,zero-fieldsplittings (cm-‘), spindensities a and phosphorescence lifetimes(set) of DBS and of biphenyl in a DBO single crystal and in a glassy ethanol solution

(lo-*

DBS DBO

gx

2.0025 2.0033

gY %

X Y

z D-.;z

E = ;(Y-X) p2 % *P

Ref. L1The “fluorine

2.0032

M) Biphenyl

Ethanol Isotropic 2.0030

DBO 2.0032 2.0033 2.0030

Ethanol Isotropic 2.0030

+0.0406

+0.0401

+0.0406

+0.0400

+0.0333

+0.0356

+0.0331

+0.0329

-0.0741 +0.1112 -0.0032 0.255

-0.0759 +0.1138 -0.0022

-0.0738 +0.1106 -0.0037

-0.0730 +0.1096 -0.0035

0.260

0.245

0.25

4.0

1.8

4.2

4.2

5

4

3

2

(0.10-0.15)

labelling method”

0.115

was used in the case of glassy solutions.

423 ESR OF PHOSPHORESCENT

BRIDGED

BIPHENYLS

We reported earlier the parameters of the lowest triplet state of the bridged biphenyls and of their fluorinated derivatives in glassy ethanol solutions. For the fluorine-substituted molecules, we have determined the spin densities on the substituted carbon atoms using the model described by Mispelter et all. These authors have concluded that the hyperfine coupling of an aromatic ;C-F fragment is related to the spin density present on the carbon atom before the substitution by

The principal values of k F are, for a unitary spin density, -24 gauss (along the C-F bond); + 193 gauss (normal to the molecular plane), and - 11 gauss (normal to the other two directions). In fluorinated molecules, a hyperfine splitting is observed in both the “Am = 1” and the “Am = 2” spectra (Figs. 3 and 4). In the latter case, the observed splitting in terms of spin densities was interpreted using a method described by Grivet 1 5. In all the cases, the results were consistent with those obtained from the “Am = 1” lines. Magnetophotoselection experiments1 6-1 8 were used in each case to assign the canonical lines to the correct molecular axes. The main results are summarized in Tables 2-4. We may conclude from them 9230

MHz

I

’

--

2000 H (gauss)

Fig. 3. Low-field parts of the “Am an ethanol glass.

= 1”

V

(r

ESR spectra of DBS and of 2,7-difluoro-DBS

in

424 -----

Dibenzothiophene 2.7-drfluorodibenzothiophene

9230

I

1450

MHz

I

1500

1550

HCgauss) Fig.

4. Hmi, lines of the DBS and of 2,7-difluoro-DBS

in an ethanol glass.

(i) All the D parameters have numerical values between 0.10 and 0.12 ’ and the phosphorescent lifetimes have an order of magnitude of a few seconds. This clearly means that all the triplet states studied here are (7r,7r*)molecular state;. (ii) The values quoted in Table 2 are very similar to the values reported by Siegel and Jude&is1 ’ for the same molecules in diethyl ether glasses. This cm-

TABLE 2 Zero-field splittings (cm-‘) biphenyls

X

D

and lifetimes (set) of the phosphorescent bridged

E

D*n

7P

s

+0.1138

-0.0022

0.1136

1.8

SO 802

+0.1136 +0.1136

-0.0022 -0.0021

0.1136 0.1137

1.6 2.0

0 NH CH,

+0.1074 +O. 1026 +0.1083

-0.0084 -0.0067 -0.0032

0.1086 0.1029 0.1078

5.8 7.2 7.0

* From Wmin line.

425 TABLE 3 Zero-field splittings (cm-‘) bridged biphenyls X

D

S

+0.1169

and lifetimes (set) of the phosphorescent fluorinated

E +0.0021

SO, 0 NH CH,(lF) CH,(2F)

+0.1054 +0.1097 +0.1119

-0.0031 -0.0033 -0.0013

D*=

TP

0.1169

2.85

0.1174 0.1095

1.5 4.5

0.1055 0.1102 0.1119

4.6 5.3 4.7

a From Hmin line.

gives evidence that the solvent has very little effect on the zero-field splittings. This has been shown by comparing the DBS parameters :A a DBO single crystal and in ethanol glasses (Table I). (iii) Tables 2-4 show that the nature of the heteroatom has little effect on the spin distribution in the triplet molecules. This is emphasized by the analogy between the triplet states studied and that of biphenyl. Thus, there is no conjugation through the C-X bond and the central bond is still of the ethylenic type as in biphenyl. (iv) Perturbations of spin densities, through an inductive effect of the atom X acting on the neighbouring carbon centres, might influence the zero-field splittings, and more especially the E values. Indeed, we observed a linear increase in the absolute E values along the series DBS, DBCH, , DBNH and DBO parallel to the atomic electronegativity scale for the corresponding X atoms: C = S < N < 0. We may also invoke a perturbation of the biphenyl geometrical structure to account for small changes of the spin distribution, particularly an in-plane rotation of the two phenyl groups with respect to the central bond. These two mechanisms would explain the observed variaTABLE 4 Hyperfine splittings (gauss) of ESR lines for glassy solutions andthe spin densities deduced (see text)

X

w&n)

aW,I

S

30.0 29.5 32.0 27.4

50.0

44.7

0.26 0.26 0.26 0.23

31.0 32.0

50.2 50.2

0.26 0.26

so2

0 C%(lF) C%(2F)

p2.7

E (lo-‘cm-‘)

t

-8 -9

10

Fig. 5. Diagram showing the variation of the E parameter versus the number n of substituted fluorine atoms at the positions 2 and 7 in biphenyl and related compounds (from ref. 4). tions of the zero-field splittings better than an increase of the conjugation through the C-X bond as stated by Siegel and Judeikisl ’. (L’) Small changes in the zero-field splittings are observed under fluorine substitution. In all cases, an increase of D and a decrease of E absolute values occur. The first effect is easily explained by the small delocalization of the n-electron density onto the Zp, fluorine atomic orbital. The positive E value of the fluorinated DBS must be noted; it was determined by magnetophotoselection. This change in the sign of E is further demonstrated by Fig. 5, which represents the variation of E values upon fluorination. This variation is qualitatively, and nearly quantitatively, the same in the four compounds in which E was measured. Its calculation would require a good theoretical model for the zero-field splittings. However, Fig. 5 suggests that the observed variation should be attributed to local effects of fluorine substitution and, more precisely, to spin delocalization onto the 2p, fluorine atomic orbital. We may thus conclude that fluorine substitution has little effect on the electronic distribution in the molecules under investigation. This would enhance the validity of the electronic local perturbation model, if necessary. (vi) The variations of the lifetime of the phosphorescence with the nature of the atom X are much weaker than expected, considering the atomic spin-orbit coupling constants of _X. This is a further argument in favour of a complete lack of conjugation through the C-X bond. Moreover, a decrease is observed upon fluorine substitution (except for DBS) as expected from an internal heavy atom effect on the spin-orbit coupling.

427 (vii) The variations of the triplet DBS lifetime with the solvent and its anomalous behaviour under fluorination are experimental evidence for the importance of non-radiative processes in the phosphorescence of this molecule.

THE PHOSPHORESCENCE

OF DIBENZOTHIOPHENE

We have emphasized in the preceding paragraphs that the nature of the atom X has little effect on the molecular properties of the phosphorescent bridged biphenyls. The lack of conjugation through the C-X bond, i.e. the lack of spin density on the heteroatom, can be accounted for if the triplet wavefunction is antisymmetric relative to the yz molecular plane. With the notation of Fig. 1, this means that the lowest triplet state of the bridged biphenyls can be denoted 3B,. The order of magnitude of the energy difference E(S, )-E(T, ) (6000 cm- ’ in the case of DBS 2o v2‘) corroborates this statement. Indeed, if we extend the Platt formalism2 2 to heterocycles, such a difference shows that T, is a 3L, state, the symmetry of which is B, in the C,, point group. Figure 6 represents the first molecular excited states of the DBS, with their symmetry assignments and energies2o l2 ‘. The splitting of T, into its

35100

‘Bl

-----

Rodiotive transittons Rodiotionless transitions

30300 E (cm’)

A2

l0.05

82

24400

0 -0.05

Fig. 6. The first molecular energy levels of DBS and the possible electronic processes of its phosphorescence (from ref. 7).

428

spin sub-levels is also represented. The notations (1) and (2) correspond to the two possible schemes of populating and depopulating the T, state. The T2 state is also represented; although it has never been observed experimentally, we know that it lies between the S, and the T, states. Magnetophotoselection experiments 4 v1’ have shown that the DBS phosphorescence is first excited by an S, +- S, absorption polarized along the y molecular axis. The intersystem crossing from S, to the triplet manifold can be conceived in two ways. (i) A IAl --f 3B, transition, symmetry-allowed through pure spin-orbit coupling, but with a great energy difference denominator. (ii) A ‘A, + 3A, transition, symmetry-forbidden at first order, allowed at second order through vibrational spin-orbit coupling, but with a smaller energy difference denominator. The main argument in favour of the second way is the fact that the exchange energy of an A, (Lb ) configuration is smaller than that of a B, configuration. In fact, it is equal to zero, at first order, in polycyclic hydrocarbons, but it must have a finite value for molecules deviating from the ideal case and, particularly, for heterocycles. This is why we think that mechanism (ii) may be excluded, though there is no experimental justification for this statement. In mechanism (i), T, will be populated via its T, spin component for symmetry reasons. The phosphorescence, which is mainly out-of-plane polarized in all the aromatics studied, i.e. 3(?r,7f*) states, will then come from the T,, spin level. The same qualitative scheme can be applied to the other compounds studied here except DBCH,, in which the order of the first two excited single states is reversed2 3. NOTE

The work presented here is part of a thesis submitted by the author in partial fuIfilment of the requirements for the degree of Docteur des Sciences _at the University of Liege. ACKNOWLEDGEMENTS

We wish to thank Drs. J.M. Lhoste, J. Mispelter and J.Ph. Grivet for many helpful discussions. We have also appreciated the interest that Prof. J. Toussaint and H. Brasseur have taken in our work. APPENDIX

The dibenzofuran used in the work reported here was contaminated by an unknown impurity (some 10B4 M). We used zone-melting refinements but we were not able to separate the DBO from its phosphorescent guest.

429

TABLE 5 Zero-field splittings (cm-’ ) and lifetimes (set) of the phosphorescent centre observed in some bridged biphenyls Matrix

D

E

Dibenzofuran Biphenyl

+0.0972 -t-O.0968

-0.0138 -0.0136

Eluorene

+0.0972

-0.0134

rP

3.7

This impurity most probably has a biphenyl skeleton, for its molecular axes coincide with those of DBS in a DBO single crystal. Moreover, it is probably substituted in positions 2 and 7: no hyperfine splitting can be resolved in any stationary line of its ESR spectrum_ F~hermore, it seems that the same impurity was described by Hutchison2 4, when he thought he was describing the phosphorescent DBCH, in a single crystal of biphenyl and in a single crystal of fluorence. Indeed, the parameters reported by Hutchison are the same as those we determined, within experimental error (Table 5). To end, we must point out that its excitation energy must be the same as that of DBS, because in a mixed crystal of DBO, the intensities of the ESR lines af the DBS and of the impurity were nearly equal. In a crystal of DBO doped with DBCHz, the impurity quenched the triplet state of the quest molecule, however. REVERENCES 1 J. Mispelter, J.Ph. Grivet and J.M. Lhoste, Mol. Phys., 21 (1971) 999. 2 J. Mispelter, J.Ph. Grivet and J.M. Lhoste, Mol. Phys., 21 (1971) 1015. 3 J. Mispelter, Chem. Phys. Lett., 10 (1971) 539. 4 J. Mispelter, J.Ph. Grivet, M. Baiwir and J-M. Lhoste, Mol. Phys, 24 (1972) 205. M. Baiwir, Chem. Phys. Left., 9 (1971) 482. : M. Baiwir, Bull. SW. Roy. Sci. Li&ge, (1971) 162. 7 M. Baiwir, J.M. Lhoste, J. Mispelter and J.Ph. Grivet, unpublished results. 8 M. Baiwir, ThBse, LiGge, (1972) 531. 9 0. Dideberg, L. DuPont and J.M. Andre, Acta Crysfallogr., Sect. B, 28 (1972) 1002. 10 CA. Hutch&on, Jr. and B-W. Mangum, J. Chem. Phys., 34 (1961) 980. 11 J-M. van der Waals and G. ter Maten, Mol. Phys., 8 (1964) 301. 12 H.M. McConnell, C. Heller, T, Cole and R. Fessenden, J. Amer. Chem- Sot., 82 (1960) 766. 13 N. Hirota, CA. Hutchison, Jr. and P. Palmer, J. Chem. Phys., 40 (1964) 3717. 14 P.J. Wagner, J. Amer. Chem. Sot., 89 (1967) 2820, 15 J. Ph. Grivet, Mol. Phys., 19 (1970) 389. 1763. 16 Ph. Kottis and R. Lefebvre, J. Chem. Phya, 41(1964) 17 J-M_ Lhoste, A_ Haug and M. Ptak, J, Chem. Phys., 44 (1966) 648,654. 18 M.A. El-Sayed and S. Siegel, J. Cham. Phys-. 44 (1966) 1416. 19 S. Siegel and H.S. Judeikis, J. Phys. Chem., 70 (1966) 2201.

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458.

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